The present invention relates in general to manually actuatable devices used to fasten a tool such as the rotor of a dentist's handpiece, and in particular to a new and useful device which comprises a push-piece and a rotary shaft, wherein the push-piece engages the shaft in response to an actuating force in order to act on tool fastening means to release a tool.
Such actuating devices are well-known. For instance, a device of this kind is used in the dentist's handpiece that is disclosed in U.S. Pat. No. 4,012,841 (which is incorporated here by reference). In this arrangement, a push-piece that is held in an outer, inoperative position by a spring, engages a rotary shaft in response to an inwardly acting force. This shaft in turn cooperates with a fastening spring provided inside a cylindrical body and the resulting unit, which forms the rotor of the handpiece, is rotatably driven at high speed by a motor. The force exerted on the push-piece is transmitted to the fixing spring to compress it. Such compression causes the inner diameter of the spring to increase thereby enabling the shank of a tool, such as that of a drilling tool, to be inserted into the spring. On releasing the force on the spring, the tool becomes fastened in the rotor. The portions of the push-piece and of the shaft that are made to engage one another are flat in the above arrangement. This produces a substantial area of contact and a high coefficient of friction between the push-piece and the shaft when they come into contact. This basically is unimportant since the push-piece only acts on the shaft when the drilling tool is being changed, while the rotor is at rest. Also, to decrease the force having to be exerted on the push-piece and hence facilitate tool changes, use is made of a soft springe to hold the push-piece in its outer, inoperative position.
A soft spring however is the cause of some drawbacks; when a dentist uses the handpiece the push-piece may inadvertently come into contact with some part of the buccal cavity. If the pressure due to this contact is such that the push-piece compresses the spring and engages the shaft which is then rotating at a speed (which may be in excess of 100,000 rpm) it will rapidly overheat because of the high coefficient of friction that exists between these two parts. Such overheating may cause a local burn on the patient and may lead to sudden and uncontrolled movements of the part of the patient. If the affected part is anaesthetized, the burn could be worse as the patient will not feel it and will have no warning reaction that will cause the dentist to move his instrument.